Failing to save a JPEG with libjpeg crashes when it uses an uninitialized jmp_buf

[smcv]

To reproduce:

• Compile SDL_image with libjpeg enabled
• Somehow get SDL_image into a state where IMG_SaveJPG_RW_jpeglib() fails. I'm currently trying to backport SDL_image 2.8.2 to Steam Runtime version 1 'scout', an Ubuntu-12.04-based environment, and for whatever reason it's failing; I'll address that eventually when I've figured out what is happening, but for now, it provides a convenient way to exercise failure code-paths that are not normally reached.
• Make IMG_SaveJPG_RW_jpeglib() fail, for example by running SDL_image's own test suite

Expected result: Saving with libjpeg fails. SDL_image catches the error and maybe tries to fall back to tinyjpeg. Either the file is saved successfully with tinyjpeg, or tinyjpeg fails cleanly too; either way, there is no crash.

Actual result: When IMG_SaveJPG_RW_jpeglib() sets jerr.errmgr.error_exit = my_error_exit, the result of a failure is a longjmp into jerr.escape, but IMG_SaveJPG_RW_jpeglib() never actually initializes jerr.escape and the result is a crash.

Suggested solution: IMG_SaveJPG_RW_jpeglib() should setjmp(jerr.escape) before it starts compression, the same as in the libjpeg loading code path.

[smcv]

Better reproducer:

$ podman pull registry.gitlab.steamos.cloud/steamrt/scout/sdk:beta
$ podman run --rm -it registry.gitlab.steamos.cloud/steamrt/scout/sdk:beta
# cd
# git clone -bSDL2 https://github.com/libsdl-org/SDL_image
# cd SDL_image
# autoreconf -fi
# ./configure --enable-tests --disable-stb-image
  (AVIF and JXL libraries are not found, this is OK)
# make
# make check VERBOSE=1